This invention relates to novel and useful improvements in chemical compositions that impart tarnish and corrosion resistance to metal articles and, in particular, relates to compositions of matter and methods for their application for treating aluminum containers to render them stain and corrosion resistant.
As is known, when metal surfaces are exposed to hot aqueous solutions, especially aluminum surfaces, for extended periods of time there is a marked tendency for such surfaces to tarnish to a brown or black coloration. Apparently, this coloration is a refractive effect of the light as it passes through amorphous platelets of generally hydrated aluminum oxides that deposit and build up on the surface of the metal. This problem becomes acute in certain industries. For example, it has been the general practice in breweries to pasteurize alcoholic beverages such as ale, stout and beer in metal containers and this is generally done by subjecting the cans to hot water baths or sprays in the range of about 140.degree. F. to about 170.degree. F. In subjecting metal containers to hot water there is a marked tendency for the metal surface, especially aluminum, to stain upon exposure and this is especially noted on the bottom portions of the containers. In practice, the severest problem of staining or discoloration is encountered during pasteurizing of the package as no organic coating is applied to the container bottom to protect it from corrosion and if left untreated, it will discolor during pasteurization, turning brownish. While seemingly this effect does not harm the contents thereof, it makes the product unappealing in its appearance to the consumer. Heretofore it has been a practice in the art to apply a so-called conversion coating to metal containers in order to suppress or passivate such metal and to prevent tarnishing as well as to promote the adhesion of inner lacquers and outer coatings of ink, paints and the like.
Various corrosion inhibitors have been used commercially such as inorganic ions including chromates and phosphates and are referred to as conversion coatings. Generally, the surface to be treated is subjected to an aqueous solution containing such ions. It is believed that these inorganic ions or heavy metal ions have the ability to bond or adhere in the form of insoluble oxides which resist tarnishing and corrosion of the metal surface. In general, the conversion coating solution is prepared and sprayed at some elevated temperature for a short period of time. After treating the metal surface with the solution, the surface is thoroughly rinsed with water to remove unreacted coating solution. However, in spite of the advantages of these corrosion inhibiting solutions, these inorganic ions have come under increasing scrutinization by environmental groups and governmental agencies with the result that there has been an increasing demand for compositions that do not have these detrimental heavy metal ions.